In recent years there have been notable developments in superconducting technology, and along with expansion in relevant fields of application the development of compact and high performance refrigerators has become essential. Such refrigerators demand light weight, compactness and high efficiency.
For instance, refrigerators with freezing cycles such as the Gifford MacMahon system or the Sterling system have been used in superconducting MRI and cryopump and the like. In addition, high performance refrigerators are indispensable for magnetic levitation trains. In such refrigerators, an operating medium such as compressed He gas flows in one direction through a regenerator filled with regenerator material and supplies the resulting thermal energy to the regenerator material, and the expanded operating medium then flows in the opposite direction and receives thermal energy from the regenerator material. In this process, as the regenerative effect is improved, thermal efficiency of the operating medium cycle is increased and it becomes possible to achieve even lower temperatures.
Cu or Pb and the like have conventionally been used as regenerator material in the above-mentioned refrigerators. However, specific heat of such regenerator material becomes noticeably low at extremely low temperatures below 20 K and consequently the above-mentioned regenerative effect does not function sufficiently making it difficult to achieve extremely low temperatures.
Therefore, in order to achieve temperatures closer to absolute zero, the use of magnetic regenerator materials which exhibit substantial specific heat in extremely low temperatures such as Er--Ni type intermetallic compounds such as Er.sub.3 Ni, ErNi, ErNi.sub.2 (See Japanese Patent Laid-Open Application No. Hei 1-310269) or ARh type intermetallic compounds (A: Sm, Gd, Tb, Dy, Ho, Er, Tm, Yb) (See Japanese Patent Laid-Open Application No. Sho 51-52378) such as ErRh is recently being considered.
However, during operation of the above-mentioned regenerators, the operating medium such as He gas passes at high pressure and high speed through gaps in the regenerator material with which the regenerator is filled and consequently the flow direction of the operating medium changes at frequent intervals. As a result, the regenerator material is subject to a variety of forces such as mechanical vibration. Stress is also applied when filling the regenerator with the material
Though the regenerator material is subject to the various forces, magnetic regenerator material of the intermetallic compounds described above such as Er.sub.3 Ni or ErRh is generally brittle and consequently is prone to pulverization as a result of mechanical vibration during operation or pressure during filling or such like. The particles generated by this pulverization influence harmfully the performance of the regenerator, such as obstructing the gas seal. Moreover, there is also the problem that the degree of deterioration in the performance of the regenerator when using a magnetic regenerator material of the intermetallic compounds as described above varies widely depending the manufactured batches of magnetic regenerator material and the like.
It is therefore the object of the present invention to provide a regenerator material which have excellent mechanical properties for mechanical vibration and filling stress and such like with a high reproducibility, a regenerator which have excellent refrigerating performance in extremely low temperature over a long period of time with a high reproducibility by using such a regenerator material, and a refrigerator using such a regenerator for extremely low temperatures.